Influence of Graphite as a Shaping Agent of Bi ... - ACS Publications

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Influence of Graphite as a Shaping Agent of Bi Molybdate Powders on Their Mechanical, Physicochemical, and Catalytic Properties Víctor G. Baldovino-Medrano,*,† Minh Thang Le,‡ Isabel Van Driessche,§ E. Bruneel,§ and Eric M. Gaigneaux*,† †

Institute of Condensed Matter and Nanosciences (IMCN), Molecules, Solids and Reactivity (MOST) Division, Universit
e catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium ‡ Department of Petrochemistry, Hanoi University of Technology, Dai Co Viet Street 1, Hanoi, Vietnam § Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium ABSTRACT: The influence of using graphite (G) as a shaping agent for bismuth molybdate (BiMo) catalysts was analyzed. Shaping was done by tableting, with addition of different loadings of graphite (0, 0.5, 1, 3, 5, 7, and 10 wt %). The use of graphite during the pressing of bismuth molybdate powders eases tableting because of the lubricating properties of the former. Furthermore, the compressive strength of BiMo-G tablets is higher than that of pure BiMo. Concerning the physicochemical properties of BiMo-G, XRD and XPS showed that graphite changes neither the relative distribution of the crystallographic phases of bismuth molybdate nor the oxidation state of bismuth and molybdenum in the tableted powders. Consequently, the shaped BiMo-G catalysts displayed similar, or slightly better, performances in the selective oxidation of propylene to acrolein. TGA analysis of samples of BiMo-G confirmed the thermal stability of the catalysts under oxidative conditions. Graphite was observed to experience crystallization into the hexagonal 2H phase during the catalytic tests. The results reported herein demonstrate that graphite is an effective shaping agent for bismuth molybdate powders.

1. INTRODUCTION The discovery of a new catalytic material is only the first step in the long path to its industrial implementation. Conventional catalytic laboratory practice is conceived to synthesize and test materials in powder form or at most as particles with sizes ranging from 250 to 500 μm. The catalytic properties of these materials are tested in microreactors in which no significant mass- and heat-transfer limitations or pressure drops occur. Conversely, industrial reactors require macroscopically shaped catalytic bodies to minimize pressure drops and plugging. In addition, powders can leach into product streams.1 Consequently, catalytic materials are shaped, most commonly either by tableting or pelletization before use in a reactor. Shaping is considered another unit operation proper to the field of ceramics engineering.2 Shaping agents comprise several substances classified as binders, lubricants, plasticizers, and compaction agents.2 These substances can be either organic or inorganic in nature. Among the most common are organic and inorganic acids (e.g. stearic acid, oleic acid, naphthenic acid, boric acid), oils, paraffins, stearates, polymers, clays, and graphite.2 Inorganic agents cannot be removed from the formed catalytic material once they have been added.2 In catalysis science, studies involving shaping are often pragmatic and aim at obtaining a good recipe whereby the added shaping agents have no marked negative impact on the performance of the formed catalysts and improve the mechanical resistance of the formed bodies. Previous research on this subject has been mostly conducted on zeolite-based catalysts because of their wide industrial use.3
8 It has been shown, for example, that the impact of clay binders on the acid properties of zeolite powders can alter their catalytic activity and selectivity.3
5,7 Other studies9,10 r 2011 American Chemical Society

have analyzed the effect of binders on the properties of mixed
vila et al.9 determined that oxides and supported metallic catalysts. A binders such as phosphoric acid and sepiolite can act on the catalytic and physicochemical properties of TiO2-supported V2O5 in different manners because they bind the catalyst particles by different mechanisms. Zakeri et al.10 analyzed the effects of using colloidal silica and ethyl silicate for shaping Co—Mn/TiO2 catalysts for the Fischer
Tropsch reaction. Both substances were reported to modify the selectivity of the catalysts after shaping. The cited works3
10 thus evidence that, apart from improving the mechanical resistance of the catalysts, the addition of shaping agents to catalytic powders can modify their functionalities. Bismuth molybdates (BiMo) are known as efficient catalysts for the partial oxidation of propylene to acrolein.11 They consist of a mixture of crystalline R-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6 phases.11 The specific role of each of these phases during the reaction is not yet fully understood.11
13 In this contribution, a first insight into the influence of the addition of graphite (G) as a shaping agent for the preparation of bismuth molybdate catalysts is presented. Graphite is most often used during shaping because of its lubricating properties and is not considered as a binder itself.2,14,15 The objective of this work was to analyze the influence of graphite on the mechanical, physicochemical, and catalytic properties of the tableted bismuth molybdate
graphite materials when used in propylene selective oxidation. Catalysts were tested in the form of both particles and Received: December 10, 2010 Accepted: March 16, 2011 Revised: March 1, 2011 Published: March 25, 2011 5467

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Industrial & Engineering Chemistry Research tablets. Characterization was carried out by different techniques, including Kr physisorption at 77 K to measure the surface area, scanning electron microscopy (SEM) to determine morphology, X-ray diffraction to determine crystallographic structure, X-ray photoelectron spectroscopy to determine the surface chemical composition and oxidation states of the metals, hardness and compressive strength tests to measure mechanical resistance, and thermogravimetric analysis to determine thermal stability.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. BiMo catalysts were prepared at the 1-kg scale by the coprecipitation method. The first step in the preparation consisted of dissolving, in a beaker, 121.25 g of bismuth nitrate (Bi(NO3)3 3 5H2O, Riedel-de Ha€en) in a solution of 0.0185 dm3 of concentrated HNO3 and 0.375 dm3 of distilled (deionized) H2O. A second solution of 33.95 g of ammonium heptamolydate [(NH4)6Mo7O24 3 4H2O, Merck] in 0.192 dm3 of H2O was prepared. With the aforementioned amounts of Bi and Mo salt precursors, a nominal Bi-to-Mo molar ratio of 1.3 was obtained. The Mo solution was added dropwise to the bismuth solution using a buret. During this stage, strong agitation of the solution was applied to provide good mixing of the two precursor solutions, thus preventing premature precipitation of bismuth molybdate. After all of the molybdenum solution had been added to the bismuth one, the obtained mixture was kept under stirring for another 30 min. Water was removed from this suspension at 353 K, giving a yellow paste. This paste was dried at 393 K and then calcined at 823 K for 3 h. Tableting of the bismuth molybdate catalyst with graphite (Merck, commercial grade) was performed after the synthesized powder had been grounded and sieved to a particle diameter lower than 100 μm and then mixed with the graphite powder. Tableting was performed in a hand-operated machine (Ateliers Ed Courtoy, series 796) in which the size of the tablets could be adjusted by fixing the separation between the two pistons that compress the powder. Cylindrical tablets of 2.3-mm length and 5.1-mm diameter were prepared. The nominal graphite contents of the prepared tablets were 0, 0.5, 1, 3, 5, 7, and 10 wt %. According to their graphite contents, catalysts are denoted BiMoxG, where x is the nominal graphite percentage; for example, BiMo-10G represents the material containing 10 wt % graphite. For x = 0, the catalyst corresponds to pure bismuth molybdate and is simply denoted BiMo. 2.2. Material Characterization. BiMo-xG materials were characterized by different techniques to determine their mechanical and physicochemical properties. 2.2.1. BET Surface Area and Mechanical Strength Measurements. The Brunauer
Emmett
Teller (BET) surface areas of the tablet materials were measured by krypton physisorption at 77 K in a Micromeritics ASAP 2010 apparatus. Samples were outgassed for at least 12 h at 423 K under a pressure of 0.4 Pa. Krypton was chosen instead of nitrogen because of the low BET surface area of the bismuth molybdate solid (SBET < 10 m2/g). The mechanical strength of the tablets was measured by evaluating their hardness and their compressive strength. Hardness tests were conducted in an Erweka TBH 200D machine using a conventional procedure. Compressive tests were carried out by placing the cylindrical tablets in a vertical position and then applying a controlled load in an Instron 5566 machine operating at constant speed (5 mm/min). The compressive strength (σε) was determined from the average of the values read in five

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measurements. The accuracy and reproducibility of these measurements were within (1 N. 2.2.2. X-ray Diffraction (XRD). Powder recovered from the crushed tablets was characterized by the XRD technique before and after the catalytic tests. XRD patterns were obtained on a D8 Br€uker Advanced diffractometer using Cu KR radiation over the 2θ range of 10
70°. The diffractometer was operated at 40 kV and 40 mA. 2.2.3. Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray (EDX) Spectroscopy. SEM
EDX characterization of the fresh tablets, to study the morphology and the qualitative chemical composition of the materials, was performed on an FEI Quanta 200 scanning electron microscope. The analysis was performed on a section of a freshly broken tablet. EDX analysis was carried out with an energy of 30 keV and following the element lines Bi L, Mo K, C K, and O K. 2.2.4. Thermogravimetric Analysis
Differential Thermal Analysis (TGA
DTA) Measurements. The thermal stability of the tablets was measured by TGA
DTA carried out with a SDT2960 apparatus from TA Instruments. For the analysis, the tablets were grounded with a spatula and heated in air to 698 K (10 K/min). Samples were maintained under these conditions for 4 h in order to determine their stability at the temperature used for the catalytic tests. Samples were further heated to 1300 K to complete the analysis. 2.2.5. X-ray Photoelectron Spectrometry (XPS). XPS measurements were performed on an SSI-X-probe (SSX-100/206) photoelectron spectrometer (Surface Science Instruments) equipped with a monochromatic microfocused Al KR X-ray source (1486.6 eV). For this analysis, fresh tablets were first ground in an agate mortar, and the obtained powder was then pressed into small stainless steel troughs mounted on a multispecimen ceramic holder. The pressure in the analysis chamber was around 1.3  10
6 Pa. The analyzed area was approximately 1.4 mm2 (1000 μm  1700 μm), and the pass energy was 150 eV. Under these conditions, the full width at half-maximum (fwhm) of the Au 4f7/2 peak of a clean gold standard sample was about 1.6 eV. A flood gun set to 8 eV and a Ni grid placed 3 mm above the sample surface were used for charge stabilization. The following spectra were recorded: general spectra, C 1s, O 1s, Bi 4f, Mo 3d, and C 1s again to check the stability of charge compensation as a function of time. The spectra were decomposed with the CasaXPS program (Casa Software Ltd., Teignmouth, U.K) with a Gaussian/Lorentzian (85/15) product function after subtraction of a Shirley baseline. Further details on the analysis conditions can be found in a previous publication.16 2.3. Catalytic Tests. The catalytic properties of the prepared materials were tested in the partial oxidation of propylene to acrolein. Catalytic tests for particles (in the granulometric fraction 250
400 μm) recovered after tableting were performed in a microreactor with an inner diameter of 0.4 cm and a length of 60 cm, using ca. 0.2 g of catalyst. For tablets, two tablets (weighing ca. 0.3 g) were introduced into a reactor with an inner diameter of 0.7 cm. A total gas flow of 0.059 dm3/min, with a composition of C3H6/O2/N2 = 2.5/2.5/95 (vol %), was fed to the reactor at atmospheric pressure. Tests were conducted at three different temperatures, namely, 648, 673, and 698 K. Reaction product analysis was performed using an online Thermo Electron gas chromatograph equipped with a flame-ionization detector (FID) and 80/100 Chromosorb and Carbowax 20 M columns in series. The measurements were performed within the time of stable activity of the catalysts. The reaction system 5468

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Figure 1. Reaction system setup for propylene partial oxidation to acrolein.

Figure 2. Pictures of pure bismuth molybdate and BiMo-5G tablets.

setup comprised a vertical glass microreactor provided with an external thermocouple attached to the reactor at the height of the catalytic bed (see Figure 1) inside an electric furnace. The temperature of the catalytic bed was directly controlled by the proportional
integral
derivative (PID) controller of the furnace. The catalytic activity is expressed as the apparent rate of acrolein (C3H4O) formation [rCw3H4O, mol/(g s)] calculated from the yield of acrolein (yC3H4O), the catalyst mass (w, g), and the total molar flow rate of reactant (F0, mol/s) using the formula rwC3 H4 O ¼

y C3 H4 O F 0 w

3. RESULTS AND DISCUSSION 3.1. Physical and Textural Properties of BiMo-xG Tablets. In this section, the physical and textural properties of the BiMoxG tablets are presented and discussed. Tableting of the pure bismuth molybdate powder was difficult to carry out because of frequent blocking of the pistons of the tableting machine. Graphite addition mitigated this problem by providing the role of a lubricant. Figure 2 shows pictures of the tablets obtained from both pure bismuth molybdenum and graphite-containing bismuth molybdate tablets (BiMo-5G). The rim of the graphite-containing bismuth molybdate tablets exhibits a thin shiny black layer. This suggests that, during powder pressing, the graphite enters into contact with the metallic walls of the piston and forms a lubricating layer that eases powder pressing. It is known that graphite aids in decreasing die
wall and interparticle friction during pressing.2 Some researchers attribute this lubricating effect to a slippage of the weakly bonded graphene layers across each other.14 Such slippage is related to the capacity of graphite to admit vapors and gases between its graphene layers, separated by 0.34 nm, thus decreasing the friction coefficient between them.15

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In addition to increasing particle size, another goal of shaping is to improve the mechanical resistance of the formed catalytic bodies. Two measurements of mechanical resistance were performed on the BiMo-xG tablets. Table 1 reports the hardness of the BiMo-xG tablets and their compressive strength as a function of the graphite loading. Graphite was found to have a negligible effect on the hardness of the bismuth molybdate tablets. Conversely, the compressive strength of the BiMo-xG tablets was markedly enhanced by addition of graphite. The σε values of BiMo-xG are more than twice as great as that of pure bismuth molybdate (3.0 MPa) for all values of x. The enhancing effect of graphite on the compressive strength of the tablets is so significant that even the lowest graphite loading (0.5 wt %) provides a significant increase. The σε value for BiMo-0.5G was 8.3 MPa. The best compressive strength for the BiMo-xG tablets was exhibited by BiMo-1G (σε = 10.0 MPa), and the lowest was exhibited by BiMo-3G (σε = 6.6 MPa). For the rest of the BiMo-xG tablets, the values of σε were very similar: 9.0 MPa for both BiMo-5G and BiMo-7G and 7.6 MPa for BiMo-10G. Therefore, tableting of bismuth molybdate can be effectively performed by the addition of a low amount of graphite (1 wt %), giving good mechanical resistance in compressive stress. The difference between the results of the hardness and the compressive strength tests can be explained considering the morphological characteristics of the tableted materials. Figure 3 presents a SEM image of the surface of a pure bismuth molybdate tablet. Particles of different morphology were observed: small round particles (diameter ca. 0.2 μm) similar to those observed previously for bismuth molybdate powders,13 piles of platelets (region 1 in Figure 3) with sharp edges (2
5 μm), some structures with rodlike shapes (region 2 in Figure 3), and others that seem to be molten bismuth molybdate particles (region 3 in Figure 3). In Figure 4a, a SEM image of the surface of a BiMo1G tablet is presented. The image shows that graphite particles (dark spots) are evenly distributed in the bismuth molybdate tablets. Similar morphological features were observed for the other BiMo-xG shaped catalysts. Figure 4b corresponds to a SEM image obtained for particles recovered after a BiMo-10G tablet had been crushed. In this figure, a platelike form of graphite with hexagonal edges is easily distinguished from the bismuth molybdate particles (whiter particles). This was confirmed by EDX examination of the chemical composition of the sample, which provided the following composition for the area featured in Figure 3b: 61 wt % C, 25 wt % O, 6.8 wt % Bi, and 7.1 wt % Mo. The morphological properties discussed above help explain why graphite does not affect the hardness of BiMo-xG tablets. The hardness test measures the resistance of a body to the penetration of a harder one. In this case, penetration was performed by the test jaw of the apparatus, which induced a shear stress on the tablet. Considering the difference in morphology between bismuth molybdate particles (Figure 3) and graphite (Figure 4b), it seems plausible to suppose that the penetrating test jaw can separate the two types of particles with the same exerted force because they are not strongly bound to each other. Therefore, the physical agglomeration of graphite and bismuth molybdate did not lead to a molecular bonding between them. On the other hand, compression resistance tests measure material resistance to a perpendicular force uniformly applied. Graphite layers are highly resistant to compression when the applied force is normal to them.15 In addition, the lubricating 5469

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Table 1. BET Surface Area, Compressive Strength, Hardness, and Weight Loss at 698 K (4 h) Registered during TGA for BiMo-xG Catalyst Tablets catalyst tablet pure graphite

a

SBET (m2/g)

compressive strength (σε) (MPa) a

hardness (kPa)

TGA weight loss (wt %)

10.2

ND

ND

ND

BiMo BiMo-0.5G

1.5 1.9

3.0 8.3

2.2 2.3

0.6 0.7

BiMo-1G

2.6

10.0

2.2

0.4

BiMo-3G

2.8

6.6

2.2

0.7

BiMo-5G

3.0

9.0

2.0

0.7

BiMo-7G

3.1

9.1

2.2

0.3

BiMo-10G

3.5

7.6

2.2

0.7

ND = not determined.

Figure 3. SEM image of a cross section of a pure bismuth molybdate tablet.

action of graphite during pressing aided the formation of more compact tablets (see Figure 2). The combination of these two effects seems to provide an appropriate explanation for the results of the compressive strength tests. In addition to being more resistant to compressive stress, the BiMo-xG tablets showed a trend of increasing BET surface area (SBET) with increasing graphite loading. Table 1 lists the BET surface areas (SBET) of the BiMo-xG tablets. All tablets were found to have surface areas lower than 5 m2/g. Bismuth molybdate catalysts normally have low specific surface areas even when prepared by methods such as sol
gel processing.13 The positive effect of graphite on SBET can be considered as a consequence of the fact that the latter has a higher surface area (SBET = 10.2 m2/g) than pure bismuth molybdate powder (SBET = 1.5 m2/g). It must be noticed that the SBET values of the BiMoxG samples are slightly higher than the arithmetic sum of the contributions of the individual components of the binary tablets. This is likely due to the fact that pure platelet crystals are stacked closely with a resulting lower specific area whereas this arrangement is perturbed in the presence of the irregular BiMo crystals. As a consequence, graphite platelets would present more exposed walls in the BiMo-xG tablets.

Figure 4. (a) Standard and (b) 10-μm SEM images of the surface of a BiMo-10G tablet.

3.2. Crystalline Structure of the Fresh Bismuth Molybdate Tablets. The relative distribution of the R, β, and γ crystallographic 5470

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Figure 5. XRD pattern of bismuth molybdate powder before tableting.

Figure 6. XRD patterns of powder from BiMo-xG tablets as a function of the graphite content.

phases of bismuth molybdates plays a central role in controlling their functionalities in propylene selective oxidation to acrolein.11,12,18 Consequently, XRD patterns of particles from BiMo-xG tablets were recorded to assess whether graphite can influence somehow the distribution of these three crystalline phases. Figure 5 shows the X-ray diffraction pattern of bismuth molybdate powder prior to tableting. The sample is composed of a mixture of the R-Bi2Mo3O12 (2θ = 29.3°), β-Bi2Mo2O9 (2θ = 27.9°, 31.9°, 33.2°, 46.7°), and γ-Bi2MoO6 (2θ = 28.4°, 32.7°, 46.9°) bismuth molybdate phases and Bi2O3 (2θ = 55.8°) and MoO3 (2θ = 23.3°, 25.7°) oxides. Among the three bismuth molybdate phases, the R-phase has the lowest concentration. Tableting and incorporation of graphite into the BiMo materials did not change the relative distribution of the molybdate species. Figure 6 features the XRD patterns for the powders recovered from fresh BiMo-xG tablets as a function of graphite

content. It can be noticed that no alteration of the relative distribution of the R-Bi2Mo3O12, β-Bi2Mo2O9, γ-Bi2MoO6, Bi2O3, and MoO3 crystallographic phases was induced by the tableting process. A peak at 2θ = 26.5°corresponding to the hexagonal graphite-2H crystallographic phase was also registered for the samples containing more than 3 wt % graphite. The intensity of the graphite-2H peak increased with graphite loading. 3.3. Effect of Graphite on the Chemical State of Bismuth Molybdate. In this section, the influence of graphite on the surface chemical state, as determined by XPS measurements, of particles recovered after crushing of the BiMo-xG tablets is analyzed. To evaluate the XPS spectra of the graphite-containing bismuth molybdate catalysts, it was necessary to employ the following strategy. First, the binding energies of the Bi 4f, Mo 3d, and O 1s peaks of the bismuth molybdate catalyst free of graphite were determined by fixing the C—(C,H) component of the C 1s 5471

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Figure 7. Decomposition of the XPS spectra recorded for pure bismuth molybdate: (a) C 1s, (b) O 1s, (c) Bi 4f, and (d) Mo 3d peaks.

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peak of adventitious carbon at 284.8 eV as a reference. This is the procedure commonly employed in the literature.19,20 Figure 7a shows the decomposition of the C 1s peak for the BiMo catalyst. The peak was decomposed into four components, all of which had the same fwhm: C—(C,H) at 284.8 eV (calibration reference), C—O at 286.3 eV, O—C—O or CdO at 287.8 eV, and COO at 289.1 eV. The C—O component was fixed at 1.5 eV from the C—(C,H) component. Figure 7b shows the O 1s peak that was decomposed into two components of same fwhm. The main one was located at 530.2 eV and was assigned to oxygen bound to bismuth or molybdenum [O—(Bi,Mo)] and to oxygen doubly bound to carbon (OdC).20,21 A complete distinction between O—(Bi,Mo) and OdC is not possible under the conditions used here. The other component at 532.2 eV was ascribed to oxygen singly bound to carbon [O—(C,H)]. Figure 7c displays the region corresponding to Bi 4f. The position of the peak Bi 4f7/2 was determined to be 159.3 eV. This binding energy has been assigned to bismuth in the Bi3þ oxidation state.20,22,23 Finally, Figure 7d features the Mo 3d region. The Mo 3d5/2 peak was centered at 232.5 eV. According to the literature, this binding energy corresponds to molybdenum in the Mo6þ oxidation state.20,22,23 When the same procedure of setting the binding energy scale by fixing C—(C,H) at 284.8 eV was applied to the catalysts containing graphite, an apparent shift of up to 0.7 eV toward higher binding energies was detected for Bi 4f, Mo 3d, and O 1s. Thus, the Bi 4f7/2 peak seemed to shift from its initial value of 159.3 eV for pure bismuth molybdate to 160.0 eV for BiMo-7G. In the case of the Mo 3d5/2 peak, the peak shifted from 232.5 eV, pure bismuth molybdate, to a maximum of 233.2 eV for BiMo7G. Such shifts do not represent a change in the oxidation state of the elements. For example, the only compound reported in the NIST database20 for Bi 4f7/2 at a binding energy of 160.0 eV is BiOCl. Such a compound cannot be present in the prepared catalysts because chlorides were not used as precursors. On the other hand, such a binding energy increase would imply the partial oxidation of the bismuth from Bi3þ to Bi4þ, a phenomenon that is unlikely to occur either during the shaping process with graphite or under the vacuum in the analysis chamber of the XPS apparatus. Such partial oxidation is known to occur in supported bismuth catalysts where a strong metal
support interaction effect is present24 or in pure Bi2O3 phases.25 For the β-Bi2Mo2O9 and γ-Bi2MoO6 phases, Uchida and Ayame22 reported that bismuth is mainly in the Bi3þ oxidation (Bi 4f7/2 at 159.3 eV) and can be reduced, under a hydrogen atmosphere, to Bi0 (Bi 4f7/2 at 157.3 eV). These authors22 also reported that the reoxidation of the bismuth molybdate samples after reduction produced only Bi3þ species. Furthermore, in a previous report, it was demonstrated that no change in the oxidation state of bismuth molybdate occurs under the oxidation environment of the propylene to acrolein reaction.13 Concerning the shift of the Mo 3d doublet, the NIST database20 reports a binding energy of Mo 3d5/2 at 233.2 eV for MoO3. XRD results (Figure 4) showed the presence of a small amount of molybdenum as MoO3. Nevertheless, molybdenum has the same oxidation state (Mo6þ) in MoO3 as in R-Bi2Mo3O12, β-Bi2Mo2O9, and γBi2MoO6. Therefore, it is hard to associate the apparent binding energy shift of the peak to a change in the oxidation state of molybdenum. Other plausible oxidation states for molybdenum, Mo5þ and Mo4þ, are expected to appear at lower binding energies, namely, 231.2 eV for Mo5þ and 229.3 eV for Mo4þ.20,22 Considering these facts, the calculated binding energies of Bi 4f, Mo 3d, and 5472

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Figure 9. Superposition of the XPS C 1s spectra recorded on BiMo-xG catalysts. Figure 8. Superposition of the XPS Mo 3d spectra recorded on BiMoxG catalysts.

O 1s for the graphite-containing catalysts were rejected. For these samples, the binding energy scale was set by fixing the position of the Bi 4f7/2 peak at 159.3 eV. This value was established from pure bismuth molybdenum samples. Figure 8 shows that, upon application of this procedure, the Mo 3d spectra recorded on BiMo-xG catalysts and on pure bismuth molybdate are all aligned at the same position with Mo 3d5/2 centered at 232.5 eV. The same pattern was observed for the O 1s peak (not shown) centered at 530.2 eV. Therefore, the oxidation states of bismuth and molybdenum in bismuth molybdate catalysts do not change after addition of graphite as a shaping agent during tableting. This is quite logical because, during the tableting process of the bismuth molybdate powder with graphite, only a mechanical mixing and further pressing of the two powders is carried out. The apparent binding energy shifts for the Bi 4f, Mo 3d, and O 1s spectra determined with the use of the C—(C,H) component as a reference are due to an overlapping of this peak with the C—C component typical for graphite. Literature reports state that the graphitic component of the C 1s peak of graphite appears at 284.4 eV.20,26 Figure 9 presents the C 1s spectra recorded on the BiMo-xG catalysts. It can be seen that a progressive shift to a lower binding energy from 284.8 eV for BiMo to 284.5 eV for BiMo-5, -7, and -10G occurs with increasing graphite loading. Furthermore, along with the binding energy shift, a noticeable change of the C 1s peak shape was observed. This is a direct consequence of the asymmetry of the C 1s peak characteristic of graphite.26 A tentative decomposition of the C 1s peak recorded on BiMo-xG catalysts was performed by modeling the C 1s peak of pure graphite and by using it as a component in the decomposition process. Other components are those characteristic of organic adventitious carbon. This allows the asymmetry of the C 1s peak of graphite to be taken into account. Figure 10 shows the decomposition of the C 1s peak for BiMo-10G. The following components were considered: C 1s of graphite (as inserted from the C 1s spectrum of pure graphite) and C—(C,H), C—O, O—C—O, CdO, and COO from adventitious carbon. Accordingly, the components were treated in the same manner as explained before. Using this procedure, the position of the graphitic contribution of the C 1s peaks was confirmed to be located around 284.5 eV, in

Figure 10. Decomposition of the C 1s peak of graphite-containing bismuth molybdate catalysts.

agreement with the literature.20,26 This corroborates the validity of the decomposition method proposed here and explains the binding energy shift of the C 1s spectra (Figure 9) recorded on BiMo-xG catalysts for increasing graphite content. The surface chemical composition of the samples (Table 2) was determined on the basis of the above analysis strategy. For the C 1s peak, the contribution of the components other than graphite, attributed to organic carbon, were added. The results presented in Table 2 indicate a constant surface Bi3þ/Mo6þ atomic ratio (1.6) for the BiMo-xG catalysts, which is in agreement with the nominal Bi/Mo ratio used during the preparation of the bismuth molybdate. The increase in graphite loading implies an increase in the concentration of surface graphite (Table 2). A particularly high concentration of surface graphite was observed for BiMo-3G (surface content of graphite = 49.2%). Recalling the observations made by SEM (Figure 3), it can be supposed that, for this sample, the area analyzed in the XPS apparatus corresponded to one in which the local composition had a higher concentration of graphite particles and a much lower concentration of bismuth molybdate particles. Moreover, a monotonic decrease was observed in the concentrations of Bi3þ and Mo6þ surface species with the concentration of surface graphite. This decrease was most prominent for the BiMo-3G sample. In accordance with this decrease, a diminution in the 5473

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Table 2. Chemical Composition Surface Analysis Results from XPS atomic % bismuth catalyst

a

Bi3þ

molybdenum Mo6þ

oxygen

carbon

atomic ratio

Bi—[O—Mo], OdC

O—[C,H]

graphite

organic carbona

Bi3þ/Mo6þ

BiMo

16.1

10.0

44.2

4.3

0.0

24.7

1.6

BiMo-0.5G

15.8

9.8

45.1

2.6

2.5

23.5

1.6

BiMo-1G

14.1

8.7

42.5

3.5

13.3

17.3

1.6

BiMo-3G

8.3

5.5

24.2

2.5

49.2

10.0

1.5

BiMo-5G

11.9

7.6

32.5

4.3

30.0

13.4

1.6

BiMo-7G

10.8

6.8

29.9

4.3

33.5

14.4

1.6

BiMo-10G

9.9

6.3

28.0

4.1

34.7

16.8

1.6

Organic carbon at. % = [C—(C,H)] þ (C—O) þ (O—C—O, CdO) þ (COO).

Figure 11. Steady-state apparent rate of acrolein formation (rCw3H4O) over particles from BiMo-xG tablets as a function of reaction temperature.

concentration of the surface oxygen linked to the metals [O —(Bi,Mo) in Table 2] was also observed. The above observations demonstrate that the use of graphite as a shaping agent for bismuth molybdate catalysts has no effect on the oxidation states of bismuth and molybdenum. As in the case of pure BiMo, the oxidation states of the two metals corresponds to those typically reported for R-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6.22 Conversely, the changes in the surface concentrations of bismuth, molybdenum, oxygen, and graphite and in the shape of the C 1s peak point out to some sort of “masking effect” of the bismuth molybdate particles by graphite. Attention must be paid to the fact that the presence of graphite cause a shift in the global position of the C 1s peak recorded on the catalysts, thus leading to the false perception of a shift in the position of the XPS spectra of the metals if the energy scale is not carefully corrected. 3.4. Behavior and Transformation of the Graphite-Containing Bismuth Molybdate Catalysts in the Partial Oxidation of Propylene to Acrolein. The BiMo-xG materials were tested in the selective oxidation of propylene to acrolein. A first series of catalytic tests was performed on the BiMo-xG particles obtained after the BiMo-xG tablets had been ground and sieved. The tests were carried out at three different reaction temperatures: 648, 673, and 698 K. Figure 11 displays the apparent rate of acrolein formation as a function of the reaction temperature for the series of BiMo-xG catalyst particles recovered from tablets

with different graphite contents. As observed in this figure, the performance of the BiMo-xG particles was at the same level as that of the pure BiMo catalyst, with the catalysts containing 7 and 10 wt % graphite even showing a slightly better catalytic performance. Therefore, depending on the amount of graphite used for tableting, this binding agent either has a positive or neutral impact on the catalytic functionalities of bismuth molybdate catalysts. The use of graphite as a codopant of Ti-doped sodium aluminum hydride has been reported to improve the hydrogenation/dehydrogenation kinetics of these materials.14 The authors of that work suggested that such an effect is possibly due to electronic effects and enhancement of hydrogen spillover mechanisms.14 It can be speculated that the same phenomenon might be playing a role in the positive effect of graphite on the rate of acrolein production. The observed positive catalytic effect of graphite is not likely to be related to the generation of new active sites on the bismuth molybdate catalyst. In fact, graphite by itself was found to be completely inactive in dehydrogenation/ hydrogenation reactions.14 Therefore, it is likely that, during the propylene-to-acrolein reaction, the active sites required for the formation of the allylic surface complex intermediate remain only those of the bismuth molybdate. The XRD results for the fresh BiMo-xG catalysts (section 3.2) demonstrated that graphite does not alter the relative distribution of the R-, γ-, and β-bismuth molybdate phases that constitute the active centers for the reaction. Evidence has been presented in the sense that the R- and β-bismuth molybdate phases constitute active centers for the oxidative dehydrogenation of the propylene molecule to generate the allylic intermediate, whereas the γ-bismuth molybdate phase would be mainly responsible for replenishing the lattice oxygen of the catalyst.12 The relative distribution of the three crystallographic phases of bismuth molybdate did not change under the environment of the reaction tests. Figure 12 shows the XRD pattern for BiMo-7G before and after the catalytic test. The same crystallographic phases of bismuth molybdate were observed in both cases. Although no changes in the crystalline structure of bismuth molybdate occurred, the peak assigned to hexagonal graphite-2H was found to significantly increase in intensity on the spent BiMo-7G catalyst. The other BiMo-xG materials exhibited XRD diffraction patterns similar to that presented in Figure 12, with the intensity of the graphite-2H peak increasing with graphite loading. Graphite can be either amorphous or crystalline (graphite-2H). In the latter, the carbon layers are arranged in an LABABL sequence, where the B layers are shifted to a registered position with respect to the A 5474

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Figure 12. Comparison of the XRD patterns of BiMo-7G before and after catalytic reaction.

Figure 13. TG analysis of particles from BiMo-xG tablets.

layers, and this is the most stable thermodynamic phase of graphite.17,27,28 The increase in the intensity of the graphite-2H peak registered for the samples after reaction can be ascribed either to an increase in the particle size of crystalline graphite during the reaction or to the crystallization of amorphous graphite. The TG analysis of BiMo-xG catalysts, presented in Figure 13, demonstrates their thermal stability under the conditions of the catalytic tests. These conditions were held (T = 698 K) for 4 h without any significant weight loss in the BiMo-xG catalysts being observed. The quantitative results of the TG analysis under the aforementioned conditions are presented in Table 1. The pure bismuth molybdate and the BiMo-0.5G catalysts exhibited complete thermal stability throughout the studied temperature range (300
1300 K). Nevertheless, for the catalysts with higher graphite contents, a significant weight loss was observed after 900 K. This is clearly associated with the decomposition of graphite, because pure bismuth molybdate was completely stable

under such conditions. From the catalytic point of view, the decomposition of graphite at high temperatures is not relevant, because selective oxidation reactions are not carried out under such severe conditions. Finally, it was decided to make a comparison between the catalytic behavior of particles recovered after crushing the BiMoxG tablets and the tablets themselves. Figure 14 shows the evolution with time on stream of the apparent rate of acrolein formation for both particles and tablets of pure bismuth molybdate and BiMo-5G at 698 K. It can be seen that the particles of BiMo-5G exhibit a slightly better performance than those of pure bismuth molybdate. This trend is reversed for the tablets, which also exhibit a slightly lower performance. The lower catalytic performance of the BiMo-xG tablets as compared to the catalyst particles can be explained if one considers that the particles have a higher amount of exposed active sites as compared to the cylindrical BiMo-xG tablets of which the rims were observed 5475

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in performing the catalytic tests. The authors also thank Michel Genet for his keen insight in a discussion of XPS results, Marc Sinnaeve for his collaboration in performing the compressive strength tests, and Inmaculada Dosuna for the pictures of BiMo-xG tablets. V.G.B.-M. thanks the office of the “Politique scientifique f
ed
erale belge” for a postdoctorate fellowship.

’ REFERENCES

Figure 14. Evolution of the apparent rate of acrolein formation (rCw3H4O) for BiMo and BiMo-5G particles and tablets with time on stream.

to be covered with graphite. These graphite rims might also be the cause of the higher activity of the BiMo tablets as compared to the BiMo-5G tablets. The graphite rim would hinder the penetration of the reactants to the inside of the tablets as compared to the case for the pure BiMo tablets, which have rough and likely more porous rims.

5. CONCLUSIONS Graphite is an effective shaping agent for bismuth molybdate powders. It significantly improves the compressive strength of the bismuth molybdate tablets, even when added in a small concentration. Furthermore, graphite itself acts as a lubricant during the shaping process. Moreover, the addition of graphite, which has a higher BET surface area than bismuth molybdate, was found to lead to an improvement in the BET surface area of the tableted bismuth molybdate powder. The addition of graphite modifies neither the chemical state nor the relative distribution of R-, β-, and γ-bismuth molybdate phases, which controls the functionalities of the catalyst in the partial oxidation reaction. Therefore, the graphite-containing bismuth molybdate catalysts displayed a catalytic performance similar to that of pure bismuth molybdate. Moreover, a slight increase in the apparent rate of acrolein formation was observed with increasing graphite loading. The graphite-containing bismuth molybdate tablets were shown to be thermally stable under the conditions of the catalytic tests, and an increase in the intensity of the crystalline hexagonal-2H graphite peak was registered in the XRD analysis of the spent BiMo-xG catalysts. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ32 10473665. Fax: þ32 10473649. E-mail: eric.gaigneaux@ uclouvain.be (E.M.G.), [email protected] (V.G.B.-M.).

’ ACKNOWLEDGMENT This work was performed within the framework of the IAP P6/17 “Inanomat” project entitled “Advanced complex inorganic materials by a novel bottom-up nanochemistry approach: Processing and shaping”. M.T.L. acknowledges financial support from the VLIR Project (ZEIN2009PR367) and thanks Nguyen The Tien, Nguyen Thi Mai Phuong and Do Van Hung, for their collaboration

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